Impact of correctable mediolateral tibiofemoral subluxation on unicompartmental knee arthroplasty implant survival in patients with anteromedial osteoarthritis

Medial unicompartmental knee arthroplasty (UKA) is advocated for treating symptomatic anteromedial osteoarthritis (AMOA). Correctable mediolateral tibiofemoral (TF) subluxation can be safely ignored according to the UKA enthusiasts. However, no clinical studies compare the results in AMOA patients with and without subluxation. This study reports the early prospective clinical outcomes of medial UKA in AMOA, with and without correctable mediolateral TF subluxation and the comparison to the retrospective larger patient cohort.


Introduction
Medial unicompartmental knee arthroplasty (UKA) is advocated for treating symptomatic anteromedial osteoarthritis (AMOA). By definition, AMOA has bone-on-bone medial compartment osteoarthritis (OA) in the presence of intact ligaments and a functionally intact lateral tibiofemoral (TF) compartment. 1 To confirm the diagnosis of AMOA, use of varus and valgus stress radiographs in addition to weight-bearing anteroposterior (AP) and lateral radiographs is recommended. 2 These radiographs are performed with the patient lying supine and the knee under study flexed at 20°, with either manual or aided stress applied to the knee. The technique is well described elsewhere. 2 A varus stress radiograph is the most reliable radiographic method to demonstrate full thickness loss of articular cartilage (bone-on-bone contact) between the medial femoral and tibial condyles. 2,3 A valgus stress radiograph helps confirm normal thickness of articular cartilage in the lateral TF compartment and demonstrates that the intraarticular varus deformity is correctable (i.e., the medial collateral ligament is not shortened). In patients with AMOA, the wear pattern on the medial tibial plateau does not extend to the posterior margin of the tibia, and in all the cases there is intact articular cartilage on the posterior margin of the medial tibial plateau. 1,2 In a proportion of patients with AMOA, a preoperative AP radiograph demonstrates the presence of mediolateral subluxation of the femur on the tibia. It is typically seen when there is significant loss of bone from the medial compartment (varus > 10°) and this subluxation persists with a varus stress view. The Oxford Group suggests that in such cases, if the valgus and varus stress views show complete reduction of the mediolateral subluxation, the presence of preoperative subluxation can be safely ignored. 4 If the subluxation persists, it is indicative of anterior cruciate ligament (ACL) deficiency and is therefore a contraindication for Oxford UKA (OUKA). 2 Although this has been advocated, no studies exist comparing the long-term results of OUKA in patients with correctable subluxation with those without any subluxation.
Does the subluxation on weight-bearing knee X-rays affect the results and outcome of UKA?
This single centre, single surgeon cohort study of consecutive cases has two aims: first, to present early prospective (P) results of OUKA in patients with AMOA between patients with correctable preoperative mediolateral TF subluxation and those without, from an independent centre; and second, to similarly compare the outcomes of a larger medium-term retrospective (R) AMOA cohort followed up by the same surgeon until 2020.

Methods
The prospective cohort (P) study comprised 272 consecutive AMOA knees (248 patients) treated with medial OUKA (Zimmer Biomet, Warsaw, USA) over a 31-month period from November 2017 to May 2020. The larger retrospective cohort (R), comprising 436 cases from May 2012 to October 2017, was then similarly evaluated according to the selection criteria for OUKA as per the published recommendations. [2][3][4] All patients underwent a standardised preoperative work-up including detailed clinical assessments and a series of radiographs as described previously. 5 Preoperative Oxford Knee Score (OKS) 6 and range of movement (ROM) was recorded. The ACL status and integrity of the lateral TF compartment were confirmed intraoperatively. If ACL was found to be friable and fragmented or absent, the patient underwent a total knee arthroplasty (TKA). In addition, the tibial wear pattern was carefully recorded using a method described by White et al. 1 The wear pattern was labelled as either anterior, or central or posterior according to the location of the deepest area of wear in relation to the AP tibial plateau dimension. All plain radiographs were assessed by an independent assessor for the presence or absence of mediolateral TF subluxation and patients were grouped accordingly into group 1 (patients with AMOA without any evidence of mediolateral subluxation on preoperative weight-bearing AP radiographs, Figures 1 a-c) and group 2 (patients with AMOA with presence of mediolateral subluxation on preoperative weightbearing AP radiographs, Figures 2 a-c). In all cases in group 2, the mediolateral subluxation completely corrected on valgus stress view with parallel reduction of the lateral joint space thereby confirming the suitability for OUKA.
To simplify the assessment of subluxation on the AP views, it was defined as any overhang of the femoral condyle (excluding osteophytes) over the medial border of the tibia. All patients underwent a cementless medial OUKA using the recommended surgical technique with Microplasty ® instrumentation with standardised postoperative management. 7 Patients were followed up at predetermined regular intervals (three months and then annually after surgery) with clinical and radiological assessment. Clinical assessment included OKS, a validated and widely used patient-reported outcome measure (PROM) and active ROM. All complications and/or further interventions on the index knee were recorded on an anonymised secure database. Any surgical intervention needed for removal or exchange of an existing implant or addition of another implant was labelled as revision.

Statistical analysis
Description of categorical variables was reported as a number and percentage. Associations between categorical variables were evaluated using chi-square (χ 2 ) or Fisher's exact tests as appropriate. Continuous variables were summarised and presented as mean and range, or as median and interquartile range (IQR). A student's t-test for normally distributed data or Mann-Whitney U test for non-normally distributed data was used to compare group differences in continuous variables. Survival analysis methods such as the Kaplan-Meier survival curves and logrank test were utilised to assess the patterns of implant survival between patients' characteristics. The multivariable Cox proportional hazards model was used to fit the predictive model for time to revision. A two-sided 5% significance level was used in all the statistical tests.

Results
The The patient demographics (Table I), implant survival rates (Table II), and clinical outcomes and complications (Table III) for group 1 and group 2 in the respective cohorts P and R are summarised accordingly. The two groups were well matched for all relevant patient demographics as well as preoperative scores, ROM, followup period and tibial wear patterns.
The complications were not associated with a specific wear pattern.

Reasons for revision
In total, six knees underwent revision at a mean of 25 months (SD = 10.9; range: 8.1-33.2 months) post base year of follow-up in the P cohort. These included two for tibial subsidence, two for bearing dislocation and two for progression of arthritis in the lateral compartment. The R cohort had 23 knees that underwent revision, and these had a slightly longer duration before revisions. The detail of the complications and procedures performed are detailed in Table III Figure 3 shows the failure patterns in the two groups for both cohorts.
Multivariate Cox regression analysis showed that presence or absence of mediolateral subluxation was a significant independent

Discussion
This is the first prospective study to assess the impact of preoperative correctable mediolateral subluxation on the outcomes of UKA in patients with AMOA. Although this is a short prospective study, it confirms the higher failure rate, contrary to the previous recommendations in the literature, when compared to those with AMOA without preoperative mediolateral subluxation. 4 AMOA knee is an ideal indication for UKA. Various studies have demonstrated the clinical effectiveness and cost-savings of UKA over TKA in such patients. [8][9][10][11][12] In a significant proportion of patients, preoperative AP radiographs reveal the presence of mediolateral subluxation. Although the exact aetiology for such subluxation is unknown, it is believed to be associated with significant wear and/or bone loss and potentially ACL deficiency -the latter being a contraindication for mobile-bearing UKA. 4 To ascertain the suitability of such patients for UKA, it is recommended that patients should be assessed using stress views -particularly the valgus stress -and intraoperative assessment of the lateral compartment as well as ACL integrity. 5 If valgus stress and intraoperative inspection confirms suitability for UKA, patients could be safely offered a UKA. In this series, we prospectively collected data on all the patients undergoing UKA. This included careful recording of preoperative patient demographics including radiological stress views, intraoperative assessment for suitability for UKA, tibial wear patterns and regular clinical follow-up at predetermined intervals. In our cohort we noted correctable mediolateral subluxation in 27% of cases. The implant survival was significantly inferior in patients with correctable mediolateral subluxation compared to those without subluxation. There were no significant differences in demographics between the two patient groups. Although the reasons for implant failure were similar in the two patient cohorts, the frequency of bearing dislocation was significantly higher in patients with mediolateral subluxation.
The assessment of wear patterns was compared to the area of wear (Table V), and no association of complications with a specific wear pattern could be found.
If we assess the risk of implant revision based on the pattern of tibial wear, it is interesting to note that the risk increases when the wear pattern changes from anterior to central or posterior. The central/posterior area of wear may contribute to pseudolaxity and higher incidence of bearing dislocation. Although the centre of the deepest portion of the wear pattern was posterior to midline in none of the cases, the wear extended to the posterior margin of the medial tibial plateau. In all cases, the ACL was functionally intact    (on intraoperative assessment), thereby confirming the patient's suitability for UKA as per established criteria. This study has certain limitations. It is a single surgeon, single centre study with a short prospective element conforming to the trend seen in the larger retrospective cohort which cannot yet be generalised. The prospective cohort also had very few failures (only six) for both groups, which could increase uncertainty in the risk estimate due to limited power. However, analysis of the prospective (P) cohort is still under continuous evaluation, and will be submitted when the five-year mean has been achieved. The study confirms association (and not causality) between higher implant failures and patients with pre-existing correctable mediolateral subluxation in patients with AMOA.
The indications for surgery, surgical technique and postoperative regimen were standardised. All patients were followed up with detailed records of their clinical outcomes and complications. The follow-up is adequate and overall implant survival is similar to other reported series with the use of cementless OUKA.
Further work is needed to establish if similar findings are observed by other researchers, and attempts should be made to understand the association between smoking status, tibial wear patterns, coronal subluxation and implant failure.

Conclusion
The AMOA with anterior wear and without mediolateral subluxation is the most suitable knee for UKA. In patients with correctable preoperative mediolateral subluxation, caution must be exercised when offering a cementless UKA.